The Origin and Evolution of Mammals - Moodle

212 THE ORIGIN AND EVOLUTION OF MAMMALS
and open-rooted so they were continuously growing.
The molars have evolved a highly specialised form,
described as zalambdodont, by losing the protocone
from the uppers and the talonid from the lowers, and
developing a V-shaped crest between the high, sharp
remaining cusps. The resulting occlusal surface
is crescentic in shape, and resembles the teeth of
several placental groups such as tenrecs and golden
moles. There is also a superficial similarity to the
zalambdodont-like molars of the marsupial mole,
although no other evidence points to a relationship of
yalkaparidonts to the latter.
Judging from the molars, the diet consisted of
soft invertebrates, but the role in feeding of the
gross incisors is hard to decipher. Archer et al.
(1991) suggested that the food consisted of invertebrates
that required puncturing by the front teeth,
but were then easy to masticate with the simple
transverse crests of the molars. Earthworms and
beetle grubs would be candidates. The postcranial
skeleton is as yet unknown, so it cannot be said
whether Yalkaparidon had the digging adaptations
that would be expected in an animal with such
a food preference.
Extinct Peramelemorphia
Some molecular evidence points to the possibility of
the bandicoots and bilbies being the sister group of
the rest of the Australidelphia, although this view
does not gain any support from their fossil record.
Apart from the possible but very doubtful peramelemorph
teeth from Tingamarra, the earliest and most
primitive known member is Yarala (Fig. 6.11(c)) from
Riversleigh (Muirhead 2000). During the Late
Oligocene and Miocene, peramelemorphs were actually
the most abundant marsupial group. There were
around a dozen species, all members of the family
Yaralidae, and they constituted the majority of the
small, mouse-sized omnivore/carnivore guild of
mammals (Archer et al. 1991). The living genera of
the modern family Peramelidae did not start to
appear until the Pliocene (Rich 1991). So far, peramelemorphs
are not represented in the fossil record
of either South America or Antarctica.
Extinct Dasyuromorphia
There are certain mouse-sized Riversleigh specimens
such as Keeura and Ankotarinja (Fig. 6.11(c))
that were considered to be primitive members of
the Dasyuromorphia (Wroe 1996), and indeed they
may well be correctly assigned to that group.
However, they are known only from jaw fragments
and isolated teeth, and Archer (1982) and
Godthelp et al. (1999) have claimed that the teeth of
these genera cannot actually be distinguished from
didelphid teeth, as is the case for the Tingamarran
Djarthia. Didelphid and basal dasyuromorph
molars both exhibit the dilambdodont condition
superimposed upon a basic marsupial tooth structure,
including a complete set of five welldeveloped
stylar cusps. Therefore, the possibility
exists that these Late Oligocene/Miocene taxa are
surviving members of a presumably paraphyletic
Didelphimorphia, from which the dasyuromorphs
had arisen. More material is required to be sure
one way or the other.
The earliest indisputable dasyuromorph, indeed
member of the surviving family Dasyuridae is also
from Riversleigh. Barinya (Fig. 6.11(e)) is well known
from a complete skull which has a number of
characters unique to the living dasyurids, and several
jaws (Wroe 1999). As a group the Oligocene–
Miocene dasyurids were less diverse than they are
today, and it has been suggested that this is related
to competition from the relatively more abundant
bandicoots and thylacines.
The Thylacinidae in contrast were more diverse
then, being represented by at least six genera in the
Late Oligocene to Early Miocene, ranging in size from
that of a cat to that of a large dog (Muirhead 1997).
Badjcinus (Fig. 6.11(f)) is the most plesiomorphic form,
with a number of primitive features of molar morphology
compared to other thylacinids, such as the
wide-angled rather than straight centrocrista and the
relatively unreduced metaconid (Muirhead and Wroe
1998). By the Late Miocene onwards diversity of thylacinids
had declined and only the modern genus
Thylacinus existed, and even it occurred only as
a single species at any one time. The family had effectively
been replaced by the larger of the dasyurids,
such as the quolls Dasyurus, and the Tasmanian devils
Sarcophilus.
The third living family of dasyuromorphs, the
numbats or Myrmecobiidae, are as yet unknown
as fossils, prior to the Pleistocene occurrence of the
living species.